Period Of Thermal Aging Research Articles

Influence of the thermal shrinkage-induced volume contraction on the yielding behavior of waxy oil

The present article investigates the effect of thermal shrinkage on the yielding behavior of waxy oil gel. The paper compares constant gap and constant normal force measurement protocols to examine the effect of measurement protocol on the rheological behavior of thermoreversible gel. The findings of this study reveal that the extent of stress overshoot and yield stress in a constant shear-rate start-up flow shows a lower magnitude in a constant gap protocol compared to a constant normal force protocol. The decrease in gel strength in the former protocol is mainly attributed to the formation of voids. These voids cause localized fractures within the crystal network. In contrast, in the constant normal force measurement protocol, gel contraction is compensated by utilizing a variable gap setting. Variable gaps compensate for the lower specific volume of the gel after crystallization, minimizing void formation and subsequent rupture of the crystal network. Hence, the gel network formed using the constant normal force protocol is more homogeneous, eliminating the uncertainties in yield stress measurement. Finally, the effect of thermal history, wax content, and aging period on the yield stress values highlights notable findings. Contrary to the conventionally accepted results, the aging period is found to impact the yield stress negatively, and a nonmonotonic relationship between the cooling rate and yield stress is noticed under the constant gap protocol. Thus, the results obtained under the constant normal force protocol are more reliable and can help in developing a fundamental understanding of the yielding behavior.

Molecular dynamics study on the influence of thermal aging on the mechanical properties of epoxy resins for high voltage bushing.

Thermal aging significantly deteriorates the mechanical properties and service performance of epoxy resins used for the high-voltage bushing. Current studies on the thermal aging behavior of epoxy resins mainly focus on experimental observations. However, an in-depth understanding of the mechanism of thermal aging of epoxy resins requires the monitoring of structural evolution of epoxy resins during thermal aging at the molecular level. To thoroughly analyze the intrinsic factors affecting structural evolution and the effect of thermal aging on the mechanical properties of epoxy resin for high-voltage bushing, epoxy resin models with different crosslinking degrees were established and thermal aging treatments at various temperatures and time periods were carried out by molecular dynamics simulation. It was found that the tensile strength of the epoxy resin was enhanced with the increase of the crosslinking degree, which was related to the elevation of the proportion of C-N and O-H bonds in its structure. With the increase of thermal aging temperature, the tensile strength of the epoxy resin decreased, which was related to the formation of weak bonds. At the early stage of thermal aging and after a period of high-temperature thermal aging, the strength of epoxy resin significantly decreases. The thermal aging of the epoxy resin is accelerated under external loading. In addition, the crosslinking degree and curing agent also affect the thermal aging resistance of epoxy resins. The results of this study can provide guidance for predicting and improving the thermal aging resistance of epoxy resins. Materials Studio was used to construct molecular models and complete crosslinking reactions. DGEBA and 44DDS (or 33DDS) were mixed at a ratio of 2:1, followed by crosslinking reaction. During this process, the Nose method was used to control temperature, the Berendsen method was used to control pressure, and the polymer consistent force field (PCFF) was used to control the motion and force of atoms. Isobaric-isothermal ensemble (NPT ensemble) was used to heat up epoxy resin models to various thermal aging temperatures of 400K, 500K, 600K and 700K. The models were maintained at these temperatures for different thermal aging times of 100ps, 200ps, 300ps, 400ps, 500ps, 600ps, 700ps and 800ps. Afterwards, the models were cooled down to 300K and subjected to uniaxial tensile testing at this temperature with a strain rate of 1 × 109s-1. The structural configurations and stress-strain data during the tensile process were recorded. The flow stress of the material was derived by counting the average stress in the 20-50% strain interval.

Effect of Si3N4 nanoparticles on IMC as well as mechanical properties of Sn58Bi/Cu solder joints during thermal aging

This paper investigated the changes in microstructure, intermetallic compounds (IMC), and mechanical properties of Sn58Bi/Cu and Sn58Bi-0.6 Si3N4/Cu solder joints in the thermal aging process at 125 °C. With the increase of the thermal aging time, the Bi-phase was gradually coarsened, but the coarsening of the Bi-phase was suppressed after adding Si3N4 NPs. The thickness of the IMC increased continuously, and some areas even detached into the solder matrix, accompanied by gradual grain growth. For the samples with Si3N4 NPs, the IMC thickness significantly decreased, and the grain size was finer. Fractures mainly occur in the Bi-rich phase of the solder matrix following a brief period of thermal aging. However, as the aging time increases, the fracture location gradually shifts from the solder matrix to the IMC, resulting in intergranular and transgranular fractures. However, the shear strength of solder joints with added Si3N4 NPs was higher during the same aging period.

The metameric effect of monolithic zirconias with varying yttrium ratios.

To evaluate the metameric disparities among monolithic zirconia materials with differing yttrium compositions across various lighting conditions. Thirty-six square-shaped zirconia samples measuring 10 × 10 × 0.5 mm were prepared from monolithic zirconia materials with three different yttrium contents. A 0.2 mm thick layer of polymerized dual-polymerizable self-adhesive resin cement was created using a silicone mold with the same dimensions as the prepared zirconia specimens. To evaluate metamerism, color measurements were conducted using a spectrophotometer device on a neutral gray background in a color measurement cabinet that offers four different illumination environments. All samples underwent aging by subjecting them to 10000 thermal cycles using a thermal cycle tester. Following thermal aging, color measurements were taken once more, and the data were recorded using the CIE L*, a*, b* color system. Two-way ANOVA and Post-hoc Bonferroni tests were employed to analyze the data. It was observed that there was no statistical difference among the color measurements made in different illumination environments of the monolithic zirconia ceramics used to evaluate metamerism (P > .05). This observation remained consistent both before and after thermal aging. After thermal aging, the color of monolithic zirconia materials exhibited a tendency towards red and yellow hues, accompanied by a decrease in brightness levels. It can be stated that different illumination conditions did not affect the metamerism of monolithic zirconia materials, but there was a color change in monolithic zirconia materials after a thermal aging period equivalent to one year.

Ultra-high molecular weight EPDM composites via crossover curing agents: Stabilized crosslink network structure and performance properties

This study investigates the impact of various crosslinking systems, such as those based on sulfur, peroxide, and their hybrid combinations, on the thermal degradation characteristics of ethylene propylene diene rubber (EPDM). During aerobic degradation, polymer networks undergo reversible physical and irreversible chemical changes. There are two kinds of irreversible chemical changes that may occur: chain scission reactions and crosslinking events. The impact of these reactions on the characteristics of the elastomer is antagonistic. When measuring the properties of an elastomer as it ages, most standard characterization techniques reveal the aggregate impact of two reactions. This work employs a straightforward approach based on compression set measurements to distinguish between the scission and crosslinking events that occur during thermal aging. This work has analyzed hybrid crosslinked materials using this method and demonstrated their remarkable thermal stability, as evidenced by their ability to maintain equilibrium between scission and crosslinking reactions. The stability of performance characteristics, such as tensile strength and crosslink density, in hybrid-cured samples during thermal aging supports the aforementioned. Furthermore, the investigation also delves into the potential of hybrid curing systems using crossover curing agents instead of coagents, which are multifunctional additives, to augment the durability of performance characteristics over an extended period of thermal aging.

Experimental study on mechanical property degradation of thermal aging laminated rubber bearing

Laminated Rubber Bearing (LRB) plays a significant role in recent bridge-supporting systems and its characteristics are mainly contributed by their nonlinear properties. However, LRB performance deteriorates during aging. To better understand the aging phenomenon and its influence on mechanical properties, several circular LRBs of different sizes were introduced. Firstly, thermal aging tests were carried out, and then pseudo-static tests were carried out to study the influence of aging on modulus, friction coefficient, and frictional sliding. The results show that the compression modulus and shear modulus are nonlinear with the aging time. The shape factor also affects mechanical properties. When the shape factor is larger, the effect of aging degradation on mechanical properties is alleviated. The frictional coefficients at the surfaces of testing LRBs get increased with the thermal aging period, which gets stable after 10-year aging. The initial friction-sliding displacements are increasing with the increase of thermal aging period.

Study on Micro Topography of Thermal Aging of Insulation Pressboard Based on Image Processing

The insulation pressboard is very important in the insulation of oil-immersed transformers, which determines the useful life of the transformer. In order to analyze the change in the micro topography of the pressboard under the condition of thermal aging in detail, the insulation pressboard samples of different thermal aging periods were made. Image enhancement, denoising, segmentation, optimization, edge detection, expansion, and corrosion were used to process SEM images of the pressboard after thermal aging, so as to extract the fiber width, hole size, and surface roughness. By comparing the relationship between the micro topography and the degree of polymerization of the insulation pressboard in different thermal aging periods, it can be concluded that when the fiber width was 91% of the unaged pressboard fiber width, and the corresponding roughness was 0.162, the insulation life was only approximately half of its original value, and when the fiber width was 79% of that of the unaged pressboard, the corresponding roughness was 0.225, and the insulation life was nearing the end of its life. This shows that the image processing method used in this paper can observe the micro topography changes during thermal aging and can estimate the thermal aging degree of the insulation pressboard.

Bondability of Pd and Au Containing Ag-Alloy Wires on Au Pads for LED Package

This study investigated the mechanism of the interfacial reaction of Ag-alloy wires in the gold bond pad of an LED die after a wire bonding process and their reliability. The results showed that in the as-bonded condition, no intermetallic compounds (IMCs) formed between the Ag-alloy wire and the gold bond pad and that the interfacial reaction mechanism was diffusional solid solution reaction. After 168 h of thermal aging at 200 °C, it was found that adding Pd to the Ag-alloy wire could improve the mechanical properties of the wire, and the bonding strength between the Ag-alloy wires and gold bond pad showed an increasing trend. It was speculated that increasing the thermal aging period allowed sufficient time for the diffusion reaction between the silver alloy and gold bond pad. The gold ball residual area difference indicated that adding Pd to Ag-alloy wire was more helpful than adding Au for boosting bond shear test (BST) strength in the as-bonded condition, while adding Au to a ternary Ag-alloy wire helped the bonding strength in terms of thermal stability. BST and wire pull test (WPT) specifications are suggested for Ag-alloy wire joints in LED packaging products based on tests of the critical quality parameters of Cpk.

Ratcheting behavior of pressurized-bending elbow pipe after thermal aging

Z2CND18.12N austenitic stainless steel elbow pipes were studied experimentally under reversed in-plane load of 20 kN and constant pressure of 17.5 MPa after thermal aged 1000 h and 2000 h respectively at the same thermal aging temperature of 500 centigrade degrees (oC). The ratcheting behaviors of pressurized elbow pipes were analyzed compared with original elbow pipe. The results indicate that the ratcheting behavior of pressurized elbow pipe is prominently affected by thermal aging. The uniaxial tensile properties of Z2CND18.12N austenitic stainless steel were also studied under the same condition of thermal aging. It is shown that yield stress of material decreases obviously with the increase of thermal aging period. Chen-Jiao-Kim (CJK) kinematic hardening model was used to evaluate the ratcheting behaviors of pressurized elbow pipes under different thermal aging periods. Simulation results are perfectly agreement with experimental data. Ratcheting shakedown boundaries of thermal aged elbow pipes were determined by CJK model.

Thermal Aging Effect on the Ratcheting Behavior of Pressurized Elbow Pipe

In this study, the ratcheting behaviors of pressurized Z2CN18.10 austenitic stainless steel elbow pipe influenced by the thermal aging process were experimentally investigated in controlled constant internal pressure and reversed in-plane bending after different thermal aging periods (1000 h and 2000 h) at thermal aging temperature of 500 °C. It is shown that the ratcheting behavior of pressured elbow pipe is highly affected by the thermal aging process. The evaluation of ratcheting behavior of pressured elbow pipe was performed using Chen–Jiao–Kim (CJK) kinematic hardening model as a user subroutine of ANSYS. The relationships of yield stress σs and multiaxial parameter χ with thermal aging time were proposed. Ratcheting shakedown boundary of aged elbow pipe was evaluated by CJK model with thermal aging time.

Thermal aging effect on the ratcheting-fatigue behavior of Z2CND18.12N stainless steel

The uniaxial tensile and ratcheting-fatigue behaviors of the Z2CND18.12N austenitic stainless steel at room temperature were studied with different thermal aging periods (from 1h to 500h) at different thermal aging temperatures (500°C and 700°C). The thermal aging process resulted in apparent changes in the ratcheting behavior and the ratcheting-fatigue life. The precipitates under different thermal aging conditions were identified by SEM observation. Considering the deterioration of the material induced by thermal aging process, aging damage factor was introduced to predict the ratcheting-fatigue life, which resulted in good prediction for all the thermal aging conditions.

Quasi-solid-state Dye-sensitized Solar Cells with Macropore-containing Hierarchical Electrodes

We investigated the influence of macropores in TiO2 electrodes on photovoltaic properties of quasi-solid-state dye-sensitized solar cells. We constructed a macropore-incorporated mesopore photoelectrode and compared the photovoltaic properties of this electrode with those of conventional electrodes containing only mesopores in the presence of a quasi-solid-state PGE. The polymer gel electrolyte (PGE) was prepared based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) with TiO2 nanoparticle fillers. The quasi-solid-state DSCs comprising the macropore-containing electrode displayed a photon-to-electron conversion efficiency comparable to that obtained using liquid electrolyte DSCs (90% of the efficiency of the liquid electrolyte DSCs), whereas the mesopore electrode displayed only 80% of the efficiency. Long-term stability measurements showed that the macropore-containing electrode maintained its efficiency during the period of thermal aging, whereas the mesopores electrode showed a significant decrease in the photocurrent density and, therefore, the efficiency. Notably, a direct observation of PGE infiltration by using a model particle revealed an incomplete infiltration of nanofillers into the mesopore. An impedance analysis confirmed that the incomplete infiltration increased the recombination rate at the electrode/electrolyte interface and therefore decreased the photocurrent density.

Impact of Ni concentration on the intermetallic compound formation and brittle fracture strength of Sn–Cu–Ni (SCN) lead-free solder joints

Cu6Sn5 and Cu3Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu6Sn5 and Cu3Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni)6Sn5/Cu6Sn5 and Cu3Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni)6Sn5 and Cu3Sn under thermal aging. The thicknesses of (Cu, Ni)6Sn5/Cu6Sn5 and Cu3Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.

Aging and condition monitoring studies of composite insulation cables used in nuclear power plants

Five composite ethylene propylene rubber/chlorosulfonated polyethylene (EPR/CSPE) insulated cables, a single composite cable with EPR/Neoprene, and a single Kerite insulated cable used in US nuclear power plants (NPP's) were evaluated. All of the cables had a CSPE outer jacket. To study the rate of degradation these cables were thermally aged at 110 and 120 degC until embrittlement of the composite insulation and/or outer jacket occurred. The degradation of the composite insulation and outer jackets was assessed by measuring elongation at break and indenter modulus. Both the indenter modulus and elongation values of the composite insulation and outer jacket exhibited excellent superposition when shifted using the time-temperature superposition principle. Regardless of whether or not the EPR insulation was bonded or non-bonded to the CSPE or Neoprene composite jacket thermal induced hardening and fusion of the composite jacket with EPR insulation was the cause for loss of flexibility of the composite wires. The aging rate of the composite insulated wires were not affected by the aging mode, namely, whether the composite wires were aged as a complete cable or with the jacket removed. However, the aging rate of the composite insulation and the outer jacket varied between different cables. Threshold indenter modulus values for successful loss of coolant accident (LOCA) performance for each of the composite insulation and outer jacket was correlated with the thermal aging periods of the studies available in the literature. Close correlation was found between the thermal aging periods used for satisfactory LOCA performance and the time to reach 50% elongation of the composite insulation. Indenter modulus values of the composite insulation and outer jacket when the composite insulation decreases to 50% elongation is also provided

Effect of wire diameter on the thermosonic bond reliability

Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6 mil and 0.8 mil diameter wires) shows formation of voids apart from intermetallic compound growth. While, with 1-mil and 2-mil diameter gold wire bonds the void growth is less significant and reveal fine voids. Studies also showed void formation is absent in the case of thicker 3 mil wire bonds. Similar tests on copper ball bonds shows good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as any voids on the microscopic scale and thus exhibits to be a better design alternative for elevated temperature conditions.

Effect of wire size on the formation of intermetallics and Kirkendall voids on thermal aging of thermosonic wire bonds

Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6- and 0.8-mil-diameter wires) show formation of Kirkendall voids apart from intermetallic compound growth. With 1- and 2-mil-diameter gold wire bonds, the void growth is less significant and reveals fine voids. Studies also showed that void formation is absent in the case of thicker 3-mil wire bonds. Similar tests on copper ball bonds show good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as no voids on the microscopic scale and thus promises to be a better design alternative for elevated temperature conditions.

熱遮へいコーティングの界面はく離挙動に及ぼす熱時効処理の影響

Knowledge of the degradation mode in thermal barrier coatings (TBCs) is a prerequisite for the prediction of their remaining life of gas turbine blades. This paper describes the effects of thermal aging on the microstructure in a YSZ-based thermal barrier coating system deposited on Ni based superalloy substrates. The heat treatment of the TBC specimens were conducted at 1373K for 100h, 500h and 1000h in air for thermal aging. The thermally aged coating system had thermally grown oxide (TGO) layer between the substrate and the TBC. TGO consisted of not only alumina but also other oxides. This oxide layer that named mixed oxides (MO) is composed of Ni, Co, Cr and O. Double cantilever beam (DCB) tests and 4 point bending tests were carried out in order to evaluate the TBC/substrate interface strength. The interface strength depended on the period of thermal aging in both type of tests. The interface strength increased initially up to the aging time of 100h, and then decreased drastically for the longer aging time. In DCB tests, crack propagated in the middle of YSZ layer for the thermally aged specimens, whereas delamination took place close to the YSZ-bondcoat interface in the unheated specimens. In 4 point bending tests, crack propagated along the interface between TBC and substrate. These results suggested that the interface strength depended on not only the formation of oxides but also the fracture properties of the top coatings.

Failure mechanism of lead-free solder joints in flip chip packages

The failure mechanisms of SnAgCu solder on Al/Ni(V)/Cu thin-film, underbump metallurgy (UBM) were investigated after multiple reflows and high-temperature storage using a ball shear test, fracture-surface analysis, and cross-sectional microstructure examination. The results were also compared with those of eutectic SnPb solder. The Al/Ni (V)/Cu thin-film UBM was found to be robust enough to resist multiple reflows and thermal aging at conditions used for normal production purposes in both SnAgCu and eutectic SnPb systems. It was found that, in the SnAgCu system, the failure mode changed with the number of reflows, relating to the consumption of the thin-film UBM because of the severe interfacial reaction between the solder and the UBM layer. After high-temperature storage, the solder joints failed inside the solder ball in a ductile manner in both SnAgCu and SnPb systems. Very fine Ag3Sn particles were formed during multiple reflows in the SnAgCu system. They were found to be able to strengthen the bulk solder. The dispersion-strengthening effect of Ag3Sn was lost after a short period of thermal aging, caused by the rapid coarsening of these fine particles.

608 熱遮蔽セラミックコーティングの界面はく離挙動に及ぼす熱時効処理の影響

Knowledge of the degradation mode in thermal barrier coatings (TBCs) is a prerequisite for the prediction of their remaining life of gas turbine blades. This paper describes the effects of thermal aging on the microstructure in a YSZ-based thermal barrier coating system deposited on Ni based superalloy substrates. The heat treatment of the TBC specimens were conducted at 1373K for 100h, 500h and 1000h in air for thermal aging. The thermally aged coating system had thermally grown oxide (TGO) layer between the substrate and the TBC. TGO consisted of not only alumina but also other oxides. This oxide layer that named mixed oxides (MO) is composed of Ni, Co, Cr and O. 4 point bending tests were carried out in order to evaluate the TBC/substrate interface strength. The interface strength depended on the period of thermal aging in both type of tests. The interface strength increased initially up to the aging time of 100h, and then decreased drastically for the longer aging time. In 4 point bending tests, crack propagated along the interface between TBC and substrate. These results suggested that the interface strength depended on not only the formation of oxides but also the fracture properties of the top coatings.

Changes in the Properties of Synthetic Fibres in Thermal Aging

Thermal aging of complex lavsan, nitron, and anid fibres and lavsan and nitron yarn was investigated in the temperature region above the glass transition temperature (120, 150, 180°C) in the free state and with shrinkage for up to 300 h. It was shown that thermal aging of chemical fibres in the general case includes periods of structural and thermochemical aging. The kinetic characteristics of thermal shrinkage of chemical fibres and yarn in the first period of thermal aging were investigated. An exponential curve of shrinkage in time was obtained and can be used to predict the behavior of fibres and yarn in the first period of aging in the free state. The kinetics of the change in the strength and relative elongation at break was investigated in prolonged thermal aging of chemical fibres and yarn in the free (with shrinkage) and fixed (without shrinkage) states. The exponential dependence of the change in the fibre breaking characteristics on the duration of heat treatment was demonstrated. The change in the properties in prolonged thermal aging in the free and fixed states in the first stage of aging takes place differently. With shrinkage, the strength decreases and the deformability increases as a result of relaxation phenomena in the first stage, followed by a symbatic decrease in the strength and deformability as a result of thermal degradation processes in the second stage. In heat treatment in the fixed state, the change in the mechanical properties in the first stage is slight, while the strength and deformability simultaneously decrease in time as a result of thermal aging in the second stage. The quantitative characteristics found can be used to predict the change in the mechanical properties of chemical fibres and yarn in conditions of prolonged exposure to heat.